You could create a spell that applied an effect that was completely opposed to the negative effect that you're trying to overwrite, and then require that the status effect from whatever spell you are trying to undo is active on your unit at the time of casting (I have no idea if it is possible to check for this, but if it is, this will work).

You would need to do some experimentation to see how, exactly, these things would stack (for instance, with an Unblind spell, whether you'd need +50% or +100% accuracy, and I don't know what you'd do for effects with variable magnitudes), if this is possible.

Alternatively, if it is not possible to create a tactical-level dispel, you could create a set of spells so that all the curse spells have opposed blessing spells (haste and slow, blindness and accuracy, wither and might, etc), designed so that if both spells were active on a unit at the same time (and, in the case of variable strength effects, had the same 'power' behind them - shards or caster levels or something else) they would negate one another. Note that if percentage effects are multiplicative, a 20% penalty counteracts a 25% bonus and vice versa, a 100% bonus would be required to counteract a 50% penalty, and so on. If the percentage effects are multiplicative, I'm also not sure that anything could be done to counter Curse, since it is a 100% penalty - if it is not something that can be counteracted in this way, you might want to modify it to something you can counteract (like 99%, if you want to keep the same feel, but to counter that you'd need a ~10,000% bonus - which I'm sure you agree is a little excessive), or make it a higher level spell (to justify the lack of a counter).